The invention relates to a surface grinding machine which has a bench for receiving work pieces, and which is reciprocable relative to a head supporting the grinding wheel by means of a drive.
In machines of this type, it is known to drive the bench hydraulically or electromechanically. When an electromechanical drive is used, the drive from an electric motor is maybe transmitted to the bench via a toothed wheel worm gear or a thread roller spindle. These known drives have the disadvantage of having a relatively small range of adjustment, a low maximum speed and too high a minimum speed. Furthermore, it is not possible to regulate the speed accurately at lower grinding speeds. In addition, the relatively large amounts of noise made by these drives is unpleasant, and servicing is difficult, particularly in the regions where the movable bench reverses. More importantly, the reciprocation requires the use of several interacting switches which, in turn, require substantial control elements. The object of the invention is to overcome these disadvantages and to provide a surface grinding machine which operates reliably even at relatively high and at low grinding speeds, with a simple control arrangement. According to the invention, there is provided a surface grinding machine comprising a head, a grinding wheel supported by the head, a reciprocating bench for receiving and supporting a work piece, a toothed belt having opposite ends thereof fixed to the bench for effecting reciprocation thereof; guide rollers mounted on the machine frame for guiding the toothed belt to a drive pinion, drive means comprising a thyristor-controlled reversible permanent magnetic field servomotor for driving said pinion, and a single control switch for controlling the said drive means, and arranged approximately in the middle of the path of travel of the bench. Two other switches, one at each end of the extent of reciprocation are safety switches, which operate in the event of a malfunction causing reciprocation to the end of travel of the bench.
A reversible permanent magnetic field servomotor drives, via transmission gearing, the toothed belt. It is controlled by the control switch.
A surface grinding machine provided with such a drive mechanism has the advantage, particularly over surface grinding machines with a hydraulic drive, of a greater range of speed adjustment. The range of adjustment can be at least 1000:1, and in some cases, even 5000:1. Other advantages are that the low grinding speed is better and more uniform, being as low as 6 mm per minute, and as high as 50 meters per minute. The lowest grinding speed may even be reduced to below 6 mm per minute, if necessary. With known surface grinding machines, speeds of from 150 mm to a minimum of 30 mm per minute may usually be obtained only with a separate crawling circuit.
Other advantages of the invention are an improved regulation of speed at low grinding speeds, almost noiseless drive, high reliability, and the fact that the drive requires almost no servicing. It is generally sufficient to check or exchange the brushes in the drive motor once a year. Servicing is also simple and convenient, since only one potentiometer is required for the grinding path and another potentiometer for the grinding speed.
The drive motor is a permanent magnetic field servomotor, preferably with eight poles and a built-in tachogenerator. The moment of inertia of this motor is very high in relation to the external moment of inertia, with the result that the machine reaches speed in a very short period of time without over-shooting. The drive motor is controlled by means of a two-pulse thyristor amplifier to which power is supplied from the power system by means of a single phase transformer. The thyristor amplifier is designed in such a way that the face grinding machine is controlled by means of a single control switch. Two other safety switches are imposed in the system for defining ends of the path of the grinding machine bench. The actual control switch is mounted in the middle of that path. The grinding path is, therefore, regulated by a potentiometer while the grinding speed is regulated by a second potentiometer. The motor is not started or stopped jerkily but by means of an adjustable phase in, such as with delayed starting and stopping. In order to avoid thermal overloading of the motor and of the surface grinding machine, the delay should not be too short.
With the drive mechanism used in the invention, the rotational movement of the drive motor is translated simply in a linear movement of the bench supporting the work pieces by means of the toothed belt engaging a toothed wheel mounted on transmission gearing connection to the drive motor.
The surface grinding machine according to the invention may be used for pendulum grinding at maximum bench speeds of up to 50 meters per minute, and optionally higher, and also for crawling grinding at the minimum speeds of, for example, 6 mm per minute or even lower. This wide range of adjustment cannot be obtained with known surface grinding machines.